The present invention relates to transformers. More particularly, the invention relates to a method and a system for estimating the conductor losses that occur in transformers during operation thereof.
Transformers are alternating current (xe2x80x9cacxe2x80x9d) devices that transfer energy from one ac circuit to another ac circuit. FIG. 1 depicts a single-phase transformer 100. The transformer 100 comprises a laminated iron core 101, a high-voltage (xe2x80x9cHVxe2x80x9d) winding 102, and a low-voltage (xe2x80x9cLVxe2x80x9d) winding 104. The HV and LV windings 102, 104 arc each wound around the core 101. The HV winding 102 can be electrically coupled to an external ac power source (not shown), and the LV winding 104 can be electrically coupled to an external load (not shown) during normal (in-service) operation of the transformer 100.
Energizing the power source causes an alternating current to flow within the HV winding 102. The alternating current induces an alternating magnetic flux within the core 101. The core 101 conducts the magnetic flux to the LV winding 104. The magnetic flux induces a voltage across the LV winding 104. The voltage across each of the HV and LV windings 102, 104 is proportional to the number of turns in the respective HV and LV windings 102, 104. The resulting current in each of the HV and LV windings 102, 104 is inversely proportional to the number of turns in the respective HV and LV windings 102, 104.
Various losses occur in the transformer 100 during operation thereof. These losses are typically classified as xe2x80x9ccorexe2x80x9d losses and xe2x80x9cconductorxe2x80x9d (copper) losses. Core losses result from the alternating magnetic flux within the core 101. More particularly, the alternating magnetic flux causes eddy currents and hysteresis within the core 101, which decrease the amount of energy transferred from the HV winding 102 to the LV winding 104. Conductor losses result from the resistance of the HV and LV windings 102, 104 to the flow of current therein (losses of this type are commonly referred to as xe2x80x9cIR2xe2x80x9d losses). These losses represent energy losses that occur during the transformation of power by the transformer 100, and can contribute substantially to the operating cost of the transformer 100.
In a competitive sales environment, the decision of a potential customer whether to purchase a transformer such as the transformer 100 is often based on the concept of total owning cost. In other words, the purchaser typically seeks the lowest combination (sum) of initial purchase price and projected operating cost over the life of the transformer. The purchaser is usually willing to pay more for a transformer having a relatively low estimated operating cost. Conversely, a transformer with a relatively high estimated operating cost will usually sell for a lower price. Hence, transformer manufacturers are subject to an economic penalty for transformers having relatively high operating costs.
Most purchasers of transformers such as the transformer 100 expect to receive a certified report from the manufacturer documenting the operating cost of the transformer, reflected in current monetary terms. Hence, transformer losses are usually measured by the manufacturer prior to shipping the transformer to the purchaser.
Conductor losses are typically measured using a so-called xe2x80x9cshort-circuitxe2x80x9d test. The short-circuit test is performed by placing a xe2x80x9cshorting barxe2x80x9d 50 (or other suitable electrical conductor) across one of the windings (typically the LV winding 104) of the transformer 100 (see FIG. 1). In practice, the shorting bar is electrically coupled to a first and a second bushing 106, 107 of the transformer 100. The first and second bushings 106, 107 are normally used to electrically couple the external load to the LV winding 104 during normal operation of the transformer 100.
A suitable ac power source 51, wattmeter 52, and ammeter 54 are electrically coupled to one of the windings 102, 104 (typically the HV winding 102) of the transformer 100 as shown in FIG. 1. In practice, the wattmeter 52 is electrically coupled to a third and a fourth bushing 108, 109 of the transformer 100. The third and fourth bushings 108, 109 are normally used to electrically couple the external power source to the HV winding 102 during normal operation of the transformer 100.
Energizing the power source 51 causes an alternating current to flow through the HV winding 102. Preferably, the voltage produced by the power source 51 is adjusted so that the alternating current flowing through the HV winding 102 (as measured by the ammeter 54) is approximately equal to the rated current for the HV winding 102. The alternating current within the HV winding 102 induces an alternating magnetic flux in the core 101. The core 101 conducts the magnetic flux to the LV winding 104. The magnetic flux induces a voltage across the LV winding 104.
The LV winding 104 is short-circuited by the shorting bar 50. The induced voltage across the LV winding 104 therefore causes a current to flow through the LV winding 104.
The wattmeter 52 measures the power delivered to the HV winding 102. The power delivered to the HV winding 102 is approximately equal to the conductor losses of the transformer 100 (including the conductor losses associated with the LV winding 104) and, in addition, the power losses associated with the shorting bar 50.
The power losses associated with the shorting bar 50 include the losses caused by the resistance of the shorting bar 50, i.e., the IR2 losses of the shorting bar 50. The losses associated with the shorting bar 50 also include the losses caused by the contact resistance between the shorting bar 50 and the first and second bushings 106, 107.
The power losses associated with the shorting bar 50 are typically included in the value of the conductor losses used to estimate the operating cost of the transformer 100. Thus, the transformer manufacturer is subject to an economic penalty, in the form of a lower purchase price, due to the inclusion of the power losses associated with the shorting bar 50 with the estimated conductor losses.
A preferred method for estimating conductor losses in a transformer having a first and a second winding comprises energizing the first winding while the second winding is short-circuited by an electrical conductor so that power is supplied to the first winding and a portion of the power is dissipated due to a resistance associated with the electrical conductor. A preferred method also comprises measuring the power supplied to the first winding, calculating the portion of the power dissipated due to the resistance associated with the electrical conductor, and subtracting the portion of the power dissipated due to the resistance associated with the electrical conductor from the power supplied to the first winding.
A preferred method for estimating conductor losses in a transformer comprises supplying power to a first winding of the transformer while a second winding of the transformer is short-circuited by an electrical conductor, and measuring the power supplied to the first winding. A preferred method also comprises calculating power dissipated by the electrical conductor in response to supplying power to the first winding, and subtracting the power dissipated by the electrical conductor from the power supplied to the first winding.
Another preferred method for estimating conductor losses in a transformer comprises electrically coupling an electrical conductor to a first and a second end of a first winding of the transformer, and energizing a second winding of the transformer. A preferred method also comprises measuring power delivered to the second winding, calculating power dissipated by resistance associated with the electrical conductor in response to energization of the second winding, and subtracting the power dissipated by the resistance associated with the electrical conductor from the power delivered to the first winding.
Another preferred method for estimating conductor losses in a transformer comprises supplying power to a first winding of the transformer while a first and a second end portion of a second winding of the transformer are electrically coupled by an electrical conductor, and quantifying the power supplied to the first winding of the transformer. A preferred method also comprises quantifying power losses across the electrical conductor in response to supplying power to the first winding of the transformer, and subtracting the power losses from the power supplied to the first winding of the transformer.
Another preferred method for estimating conductor losses in a transformer comprises energizing a first winding of the transformer by supplying power to the first winding while a second winding of the transformer is short-circuited by an electrical conductor, and measuring the power supplied to the first winding. A preferred method also comprises adjusting the measured power to account for power losses associated with the electrical conductor in response to the energization of the first winding thereby providing an estimate of conductor losses in the transformer.
A preferred embodiment of a system for estimating conductor losses in a transformer comprises a power supply for energizing a first winding of the transformer, a wattmeter for measuring power supplied to the first winding, an ammeter for measuring a current in the first winding, a voltmeter for measuring a voltage across an electrical conductor electrically coupled to a first and a second end of a second winding of the transformer, and a computing device.
The computing device comprises an input/output interface for communicating with the power supply, wattmeter, ammeter, and voltmeter, and a central processing unit electrically coupled to the input/output interface. The central processing unit comprises a processor electrically coupled to the input/output interface, a memory-storage device electrically coupled to the processor, and a power supply electrically coupled to the processor and the memory-storage device.
The central processing unit also comprises a set of computer-executable instructions stored on the memory-storage device for causing the power source to energize the first winding, causing the wattmeter to measure the power supplied to the first winding, causing the ammeter to measure the current in the first winding, causing the voltmeter to measure the voltage across the electrical conductor, calculating power dissipated by the electrical conductor in response to energization of the first winding based on the current in the first winding and the voltage across the electrical conductor, and subtracting the power dissipated by the electrical conductor from the power supplied to the first winding.